After entering the eyes, light passes through the lens and reaches the retina. Light will cause photoreceptor cells known as rods and cones to hyperpolarize. The photoreceptors synapse with other cells and eventually output is transmitted to the optic nerve.
The signal generated by the retina in response to a flash of light is known as the electroretinogram. This response may be recording using conventional neurophysiological equipment and reflects overall retinal function.
The optic nerves from each eye progress medially as they pass into the vault of the skull and shortly reach the optic chiasm. It is along the course of the optic nerves that nerve action potentials may be measured intraoperatively. At the optic chiasm fibers from the temporal portion (nasal visual field) of the retina continue on ipsilaterally into the optic tract, while the nasal portion (temporal visual field) crosses the midline and enters the contralateral optic tract. The optic tract fibers then enter alternating layers of the lateral geniculate nucleus. Axons then project toward the cortex, thus forming the optic radiations, and eventually reaching the occipital cortex. It is this visual cortex which is responsible for generating the major components of what is commonly referred to as a visual evoked potential (visual evoked response).
The majority of routine clinical evoked potential testing is performed upon awake, cooperative subjects using an alternating checkerboard pattern as the stimulus. In order to be an effective stimulus, the subject must keep his eyes open and focus on the changing pattern. In those subjects who cannot cooperate, the alternating checkerboard pattern is not useful. Common examples of such subjects include the mentally impaired, young children, and those under the influence of anesthesia. Frequently a flashing light is used in order to obtain a response in such individuals. Additionally a flash is the typical way of eliciting an electroretinogram in order to assess retinal function.
It has proven to be an exceptionally difficult task to employ such flash visual evoked responses in the operating room. A traditional strobe light is not feasible as it would be too distracting to the operating room staff and difficult to aim at the subject's eyes. Therefore reusable goggles with light emitting diodes have been employed. Unfortunately, the results obtained with such equipment has been sub-optimal. Problems include: 1.) A reusable device must be cleaned in between patients 2.) Tightly fitting goggles pose a risk of damaging the eyes 3.) Goggles may fall off the eyes or move intraoperatively and be difficult to reposition once the procedure is underway 4.) The light emitting diodes utilized, tend to be too weak to produce an adequate stimulus.